An apparatus, for example as shown in FIG. 12, has been conventionally known as one of the optical information recording and reproducing apparatuses as described above.
In this apparatus, a convergent light 101 for an information recording or reproducing operation is irradiated through an optical system 102 to a recordable surface formed on an optical recording medium 100, and then a reflected light from the optical recording medium 100 is detected by a photodetector 103 such as a photodiode or the like to perform the information recording and reproducing operations. In a case where an information has been already recorded on any one of tracks formed on the optical recording medium 100 and the track is now indicated to reproduce the information recorded on the track, a seek operation for moving an optical head 104 to the track is carried out as follows. The optical head 104 is moved so as to traverse the tracks on the optical recording medium 100, and a signal indicating a traverse motion of the convergent light 101 for the reproducing operation over a track (hereinafter referred to as "track-traverse motion" for each track) is generated and detected every track. The number of track-traverse signals thus obtained is counted every track-traverse motion of the optical head 104 (convergent light 101), and on the basis of the counted track number, the optical head 104 is seekably moved to the desired track.
FIG. 13 shows a conventional speed control device for an optical head which controls a moving speed of the optical head in the seek operation. In this type of the speed control device for the optical head, a convergent light reflected from the optical recording medium is detected by a photodetector of the optical head 104, and is supplied to a tracking error signal detection circuit 105 in which a tracking error signal as shown in FIG. 14(a) is outputted in response to the occurrence of a tracking error. The tracking error signal 106 from the tracking error signal detection circuit 105 is input to a track-traverse detection circuit 107 to output a track-traverse signal 108 as shown in FIG. 14(b). The track-traverse signal 108 is a pulse signal whose logical state (0 or 1)" is inverted every time the tracking error signal 106 passes a zero-cross point as shown in FIG. 14(b). That is, the track-traverse signal 108 represents a track-traverse motion of the optical head 104.
The track-traverse signal 108 thus obtained is input to a track count circuit 109 to count the edge 110 of each pulse (pulse edge) of the track-traverse signal 108 as shown in FIG. 14(c), thereby calculating the number of residual tracks arranged between the current track and the desired (destination) track and outputting a signal 111 representing the number of the residual tracks (hereinafter referred to as "residual track number"). The signal 111 representing the residual track number is input to a reference-speed conversion ROM 112 to output the digital signal of a reference speed 113 as shown in FIG. 14(d) which is assigned to the calculated residual track number, the residual track number serving as an address to the desired track (destination track). Here, the reference-speed conversion ROM 112 is designed so as to output the digital signal of each reference speed every shift from one track to another.
The reference speed signal 113 from the reference-speed conversion ROM 112 is input to a D/A converter 114 to be subjected to a digital-to-analog conversion, thereby obtaining an analog value 115 corresponding to the reference speed signal 113. The analog value is subjected to a zero-order hold until the edge of a next pulse of the track-traverse signal 108 is detected. The reference speed signal 115 as shown in FIG. 14(e) which has been subjected to the zero-order hold, is supplied to the inverse terminal of a differential amplifier 116.
The track-traverse signal 108 from the track traverse detection circuit 107 is also input to a clock count circuit 117 to calculate a time interval between the edges of the successive pulses of the track traverse signal 108 on the basis of a count value of clocks which are outputted from a reference clock circuit 118. That is, the time interval of the successive edges of the track-traverse signal 108 is represented by the count value (the number of counted clocks) from the reference clock circuit. The counted clock number 119 thus obtained is input to a relative-speed conversion ROM 120 and subjected to a inverse-conversion with the count clock number 119 being an address, thereby obtaining a relative speed for the track-traverse motion. The relative speed 121 for the track-traverse motion thus obtained is converted into an analog signal 123 in a D/A converter 122 and then is input to the non-inverse terminal of the differential amplifier 116.
A different between the reference speed signal 115 and the relative speed signal 123 for the current track traverse motion is amplified by the differential amplifier 116, and an output signal 124 from the differential amplifier 116 is supplied through a switching circuit 125 and a driver amplifier 126 to a head actuator 127 for driving the optical head 104 to the desired track at a prescribed speed through a speed servo loop.
The switching circuit 125 is supplied with the residual track number signal 111 of the track count circuit 109 to detect that the residual track number for the seek operation is equal to zero, and on the basis of the detection result switches an input source to the driver amplifier 126 from the differential amplifier 116 to a tracking servo circuit 128. The tracking servo circuit 128 comprises a circuit for performing a phase-compensation and gain-adjustment, and is one of the constituents for the tracking servo loop. Accordingly, the tracking servo circuit 128 is used in combination with the other constituents such as the driver amplifier 126, the head actuator 127, the optical head 104 and the tracking error signal detection circuit 105 to accurately move the optical head 104 to a desired track on the optical recording medium 100.
As described above, the moving speed of the optical head 104 is determined by a time interval for the track-traverse motion of the optical head 104 over each track, and thus hereinafter "speed" means a relative speed between the optical head 104 and a current track to be currently traversed.
However, the conventional speed control device for the optical head as described above has the following disadvantages. That is, the conventional device is so designed that a track-traverse motion is detected on the basis of a pass of the tracking error signal 106 over the zero-cross point thereof and a reference speed signal 115 assigned to a residual track number is produced and outputted by the reference-speed conversion ROM 112 using only the track traverse signal 108 which is produced in accordance with the passage of the tracking error signal 106 over the zero-cross point. Therefore, the reference speed signal 115 outputted from the reference-speed conversion ROM 112 varies stepwisely as shown in FIG. 14(e). In order to perform a seek operation at high speed, it is ideally preferable that accelerating and decelerating operations of the optical head are carried out in uniformly accelerating and decelerating modes (motions) respectively, that is, a speed variation with time is constant as shown in FIG. 14(f). However, when the reference speed signal 115 varies stepwisely as described above, it is difficult to perform the accelerating and decelerating operations at the uniformly accelerating and decelerating modes, and particularly a difference between the stepwisely-variable reference speed signal 113 and a speed to be actually controlled is larger at a low speed range. Therefore, the head actuator 127 for driving the optical head 104 can not be uniformly decelerated in the seek operation, and thus the driving of the actuator 127 is delayed with respect to that in an ideal decelerating operation (that is, an uniformly-decelerating operation). This delay of the head actuator 127 (or the deviation of the actual decelerating operation from the ideal decelerating operation) causes the seeking operation to be remarkably deteriorated.
Accordingly, the actual speed variation of the optical head is made with delay or deviation from the ideal speed variation at the uniformly-decelerating mode in the low speed range as show in FIG. 14(f), and in addition is vibrated when a loop gain is high. As a result, there occurs a case where even if the reference speed signal 113 is set to zero at a track just preceding to the desired (destination) track (a track whose residual track number is 1) as shown by (i) of FIG. 14(f), the moving speed of the optical head which has just traversed the center of the desired track is larger than the maximum moving speed of the optical head below which the optical head can be subjected to a tracking servo operation to accurately pull or seek the optical head onto the desired track (hereinafter referred to as "a seekable speed").
In this case, even though a switching or selecting operation of switching to or selecting the tracking servo circuit 128 is carried out by the switching circuit 125 at a time point of (i) of FIG. 14(f), the optical head can not be pulled onto the desired track, but overruns the desired track as shown by (j) of FIG. 14(f). In addition, even in a case where a residual speed at the instantaneous time when the tracking servo operation is selected by the switching circuit 125 is slightly larger than the seekable speed at which the optical head can be surely subjected to the tracking servo operation (that is, the optical head can be surely pulled onto the desired track) as shown by (h) of FIG. 14, positive and negative feedbacks which are ordinarily used to perform the seeking operation are countervailed because they are alternately carried out every half period of the tracking error signal, so that these feedbacks do not contribute to a decelerating operation of the optical head. Therefore, the overrunning of the optical head is continued until the residual speed is decelerated down to the seekable speed at which the optical head can be surely pulled onto the desired track as shown by (h) of FIG. 14, and thus a seek error indicating an overrunning of several or more tracks occurs.
In order to overcome the above disadvantage, the following two techniques have been conventionally adopted in the conventional speed control devices for the optical head. One is a technique that variation of the reference speed signal in the low speed range is set to a small value to restrict a control band (range) for the speed-servo operation, and the optical head is slowly approached to the desired track. The other is a two-step access technique that a concise speed control in the low speed range is intentionally abandoned, however, a rough seek operation of roughly moving the optical head near to the desired track is once carried out and then a concise seek operation of accurately approaching the optical head to the desired track is carried out.
The former has the following new disadvantages. A long time is required to perform a speed control in the low speed range because the variation of the reference speed signal in the low speed range is set to a small value, and in addition it is impossible to carry out a uniformly-decelerating speed control. Likewise, the latter has the following new disadvantages. Since the two-step speed control operation including rough and concise seek operations is required, the control device is complicated in construction and high in cost. In addition, a long time is required to perform the speed control.
In order to overcome the above disadvantages, there has been proposed a brake pulse supply technique in which just immediately before the seek operation is finished, the speed control is opened and a brake pulse is supplied to rapidly reduce the moving speed of an optical beam (optical head). This technique is disclosed in "National Technical Report", Vo. 35, No. 2, pp 67-73, April 1989.
As shown in FIG. 22, in this brake pulse supply technique, the optical beam is moved in a closed-loop speed control mode until the optical beam reaches a prescribed point X, and moved in an open-loop speed control mode after the optical beam exceeds the point X. When the optical beam arrives at the point X, a CPU calculates a current position of the optical beam on the basis of the period of a track(groove)-traverse signal just before the point X, and calculates such pulse height and pulse width that the speed of the optical beam at a desired track position Z to be sought is equal to a predetermined speed. Signals representing the pulse height and pulse width thus calculated are supplied to the actuator to forcibly stop the optical beam on the desired track and then starting the tracking servo control operation. As a result, this technique enables a seek time to be shortened.
However, this brake pulse supply technique carries out, in the open loop mode, the speed control just before the completion of the seek operation, and therefore in consideration of a speed variation or the like due to disturbance, the moving speed of the optical beam can not be necessarily decelerated to the desired speed at a time when the brake pulse is supplied, and this failure of the deceleration of the moving speed would induce occurrence of a seek error. In order to overcome this disadvantage, it seems indispensable to utilize in combination a learning control technique or the like in which a correlation between the moving speeds of the optical beam at the instantaneous time when the brake pulse is supplied and the moving speed of the optical head after the brake pulse is supplied is learned and the pulse height (peak-to-peak value) and pulse width of the brake pulse are controlled by the CPU. However, like the conventional techniques as described above, this technique also has the following disadvantages. That is, the construction of the control system is more complicated, and other techniques such as a concise seek technique and so on must be used if the learning control technique is not adopted.